1. Field of the Invention
The invention relates to a resilient positioning apparatus, that is an extensible and retractable telescoping apparatus designed to position a work piece accurately, and yet be capable of withstanding shock loads from the work piece contacting the apparatus.
2. Prior Art
The present invention can be used in the sawmill industry, and is particularly adapted for positioning cylinders associated with log positioning knees of a log carriage. Positioning apparatus are commonly hydraulic cylinders and these have been used in diverse industrial applications. Sometimes the cylinders are subjected to shock from the work piece contacting structure associated with the cylinder and such shocks can damage, or at least lessen accuracy of, the positioning cylinder. This is particularly true of the positioning cylinders used in a sawmill log carriage. To the inventor's knowledge, there are two main possible approaches to protect the positioning cylinder from such shocks, and these are discussed briefly below.
The first approach might be to resiliently mount the cylinder, for example, by using a mechanical compression coil spring or other resilient device mounted in tandem with the cylinder, ie. at an end of the cylinder remote from the end experiencing the shocks. This can be termed a tandem, stacked or compound positioning device and might be adequate for light loads, but to provide sufficient travel for the cylinder to absorb typical shock loads from heavy logs, the spring must possess considerable resilient, and for a coil spring, this requires a considerable length. Thus an excessive amount of space would be required behind the positioning cylinder which is usually inconvenient or impossible on a sawmill carriage. Furthermore, the cylinder and tandem spring device would form an excessively long combination which might be subject to buckling under high compressive loads. In some compound or tandem spring combinations, permanent deformation or wear of the springs may produce slackness or lost motion, which reduces accuracy of re-location of the positioning cylinder. Thus accuracy would likely decrease with much use of the device. Also, excessive "working" of the spring under load could result in premature fatigue failure of the spring.
The second approach to protecting the cylinder from shock is to provide the hydraulic circuit which is used to position the components of the positioning cylinder with pressure limiting valves. These vales open automatically when fluid pressure exceeds a pre-determined amount due to excessive force on the positioning cylinder, and then close when pressure returns to normal, the hydraulic power supply then repositioning the components. Whilst such devices can be adequate in some applications, in sawmill carriages difficulties commonly arise. Each collapse and recovery of the apparatus requires operation of valves, a hydraulic power supply and various other components, which determine accuracy of repositioning, and thus wear accumulates after the apparatus has sustained many shocks. Also the hydraulic power supply is required to replace fluid to reposition the cylinder, thus incurring greater energy requirements. Thus shock absorbing circuits of this type tend to be relatively complex and are known to be troublesome for servicing, due to rapid wear of initial valves, etc. resulting in a gradual loss of accuracy. Also, response of the hydraulic circuit can be such that the cylinder is repositioned relatively slowly after displacement due to the shock.